Auto enthusiasts have long produced cars known as “hot rods” by modifying the engines of American cars to increase their speed and acceleration. Hot rods used in drag racing generally have open headers, i.e., individual exhaust pipes that run directly from the engine to the atmosphere without a muffler, often referred to as “zoomies.” While this type of exhaust system provides optimal performance, due to the minimization of backpressure, the lack of a muffler generally makes such exhaust systems unacceptably loud for street use. Many hot rod owners would like their vehicles to be “street legal,” i.e., in compliance with noise and emission laws and ordinances, while still having the option of open zoomies for use at the drag strip.